Biobased Thermoplastic Elastomers Derived from Citronellyl Glycidyl Ether, CO₂, and Polylactide

Biodegradability and biobased feedstocks are key requirements for sustainable materials. This work presents the synthesis of PLLA-b-PCitroGEC di- and triblock copolymers [PCitroGEC: poly(citronellyl glycidyl ether carbonate)] as degradable thermoplastic elastomers (TPEs), sourced from biorenewable feedstocks. l,l-Lactide (LLA) is produced by the fermentation of corn or sugar on a large scale, while citronellol can be extracted from rose or lemon grass. A key feature of the current TPE structures is their low glass temperature (T-g) of the highly flexible polycarbonate midblock, based on PCitroGEC. The latter was synthesized by catalytic copolymerization of CO2 and citronellyl glycidyl ether, using (R,R)-(salcy)-Co(III)Cl (CoSalenCl) and bis(triphenylphosphine)iminium chloride ([PPN]Cl) as a catalyst system. The resulting PCitroGEC macroinitiators (11,000 to 26,000 gmol(-1), DMF) were used in a DBU-catalyzed ring opening polymerization of LLA, resulting in a series of PLLA-b-PCitroGEC triblock copolymer structures. Molar masses range between 18,000 and 41,000 gmol(-1), with the molar fraction of the "soft" PCitroGEC block varied between 22 and 60 mol %. Glass temperatures of the block copolymers were studied with a combination of temperature-modulated differential scanning calorimetry and dielectric spectroscopy techniques. Small-angle X-ray scattering (SAXS) confirmed nanophase separation for all synthesized TPEs. SAXS was further employed to construct the PLLA-b-PCitroGEC-b-PLLA phase diagram. It comprises classical phases (spheres, cylinders, and lamellae). Tensile testing illustrated elastic properties for all TPEs with elongation at break up to 600% and almost no plastic deformation for copolymers with a PCitroGEC content above 31 mol %. Cyclic tensile tests confirmed the elastic recovery properties of the TPEs. Furthermore, the materials exhibited low E-moduli of 0.15-1.0 MPa, rendering PLLA-b-PCitroGEC-b-PLLA triblock copolymers suitable for potential use in soft tissue engineering.